Hydrated Lime Mix for Exterior Wall Plaster

Walter from LandTrees.net in Panama shares with us his best outdoor lime plaster technique.

Hydrated lime plaster mix for exterior walls
Hydrated lime plaster mix for exterior walls

“The best experience we’ve found for outdoor plasters is using 90%-91% dehydrated lime. We fill this hydrated lime in blue drums (55gal. drums) and add water to a ratio of about 1 kg (2.2 pounds) to 1 gal. of water. We re-stir this lime/water mix every day during the next 4-5 days. After that we only check to ensure the mix is always soaked in water (the top lime layer should not be exposed to air. it should always remain a few inches under water). When we plaster we take this lime soaked in water and mix with sand and add water as may be needed.

The mix that best worked for us for outdoor wall plaster is a ratio 3x sand to 1x lime. In our tropical region with high humidity this mixture will take forever to dry. To expedite the drying time we add some cement to this mix, normally in the region of 20% of the weight of the lime (or 5% of the entire sand/lime weight). In dryer climates this should not be necessary as the lime plaster should dry fine in a timely manner (not to get washed off by rain before drying).

Our “Majestic El Chorrera Dome” was covered with the above lime plaster mix 2 years ago after suffering various other wall plaster mixes (all failed). The Dome is now withstanding its 3rd rainy season and till date we have not seen any problems and we have not even applied the last and finishing layer yet…”

Source: LandTrees.net
LandTrees YouTube channel

6 thoughts on “Hydrated Lime Mix for Exterior Wall Plaster”

  1. Hello,
    I am in the process of mixing water, DRY hydrated lime powder and fine sand for interior wall plaster inn old house.
    Your recipe states

    ” mix that best worked for us for outdoor wall plaster is a ratio 3x sand to 1x lime..”

    Now sand is MUCH heavier than the powder lime. So I inquire of you:
    Do you mean parts by WEIGHT, or parts by VOLUME ?

    I am guessing weight, since three times the volume of the lime for the sand seems out of bounds.
    – John

  2. Hi Owen,

    This seems like a really sturdy example of a lime plaster, and I’m considering this as an external plaster for our house. In your opinion, if I were to use a magnesium cement mix-in instead of what I’m assuming here was portland cement, would it perform in the same way (though better, given MgO cement’s superior qualities)? Since MgO cement tends to set much faster, would it make the drying time too fast to be workable? If you think this route is worth pursuing, I plan to perform some experiments.

    If there’s a better plaster option in our climate (mostly mild, with hot summers, occasionally sub-freezing winters, and fairly humid at times throughout the year), especially if I could use MgO cement in it (or a completely earthen mix, perhaps stabilized with lime), I’d appreciate the suggestion.

    Thanks so much!


  3. Interesting topic “Painting Earthbags”. We at times paint our Earthbags with a Lime paint. In our part 2 Video we show our “Air Tower” where we only applied a protective Lime paint layer. It did not hold up under our rainy seasons and vast parts blistered or ripped off as can be seen on this part 2 Video.

  4. I was wondering, if you simply paint the earthbags, would they still deteriorate? I understand they wouldn’t be protected from tears and such.

    • Polypropylene bags can last for centuries if protected from sunlight. So yeah, paint the bags and they should last a very long time. This is best for large jobs or where the project will take a long time until the walls are plastered.


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